Something like that. They would surrender to noobs sized force too if they where small enough, don't know the rules but there's speedrun type video(s) out there where a guy gets a castle to surrender very early because it had just changed hands, of course the only goal was getting a castle fast, no plans for continuing the game.
In bannerlord what comes up for me is say the garrison sallys out and is defeated, the routed units actually go back into the garrison and you have to finish siege camp and then go fight them, even though it's a tiny force. I would make sense for them to surrender. Like wise if you attack and retreat in a siege leaving a small number of enemies, surrender would seem in their favor. Surrendered troops should go strait into the garrison/militia IMO. I think enemies that route during the final siege actually do this now but I haven't confirmed it. All I know is in "solo seige" when I make the enemies rout by sniping them I always end up with surprises in the garrison and tiny militia